This invention relates to intrusion detector systems and more particularly to systems using geophone or seismometer detectors connected together in a noise cancelling network configuration.
A geophone or seismometer is a vibration sensitive device which may be implanted on or in the earth to generate electrical signals when the earth moves. Therefore, if an intruder enters a geophone protected area, the resulting earth movements shake these devices and they generate an electrical signal to give an alarm.
A difficulty with this type of system is that it also responds to natural vibration sources, such as earth tremors, thunderclaps, and the like. Since natural sources move all geophones coherently, the resulting signals are additive. Thus, it is necessary to discriminate between the local disturbances which are caused by an intruder and the wide area general disturbances, which occur responsive to natural causes.
Historically, geophones have been connected in arrays where an in-phase excitation or vibration produces additive signals, as several detectors are shaken in-phase. This system is satisfactory when the source of earth vibrations is a signal which is additive, as when the vibrations emanate from the interior of the earth. These types of arrays are also able to cancel horizontally propagating signals, when the signals are within a frequency band which is subject to an out-of-phase addition in the array.
Unfortunately, an array of geophones which is connected additively produces an overwhelming noise because all geophones hear the same noise. The noise adds while cancelling or absorbing the desired individual detector output. While an array which is additively connected can cancel end-on noise travelling along the length of the array, the directionality of the array is contrary to the usual deployment needs for intruder detectors.
Arrays almost always lie along the perimeter of the area which is to be protected. Often the perimeter is also close to a public road where traffic produces noise which activates the array. Thus, an undesired noise is augmented by the many in-phase detectors. Such a system is not optimized, even when offsetting "noise-cancelling" detectors are set out. A parallel array may be offset, parallel to the primary array in order to give a 3 dB noise-cancelling effect at a given frequency. However, such an offset system can not salvage an otherwise poor situation. Meanwhile, the array produces large noise signals, as when a thunderstorm produces lightning and thunderclaps which drive all detectors in-phase, as the air shock strikes the ground.